Posted by Matt
As promised long ago, here's another post to help provide some physics background for those of you who are not physicists. I'll briefly summarize the structure of the Standard Model of particle physics. This will hopefully make some of the other things I want to write about later more comprehensible.
In physics, we know of four basic "forces" in nature: electromagnetism, the weak force, the strong force, and gravity. Gravity is the odd one out, and I won't elaborate on it now.
Read on for further background:
The other three forces are all carried by particles of spin 1; you may have read popular science books that describe how one can think of the forces as being transmitted by messenger particles. These spin 1 particles are called "gauge bosons." In general, a "boson" is a particle with integer (i.e., whole number) spin (0, 1, 2, and so on). If you're unfamiliar with the concept of spin, this isn't so helpful a thing to say. The thing that it is easy to give a sense of without mathematics is the "personality" of bosons: they like to cling together in groups. There's no limit to how many of them can sit in a particular state, so they will tend to all pile in together. This is as opposed to fermions, particles of half-integer spin (1/2, 3/2, and so on), which obey the Pauli exclusion principle that says no two of them can be in the same state. Think of bosons as gregarious and popular, and fermions as loners. It may seem reasonable that no two things can be in the same place at the same time, but from a mathematical standpoint, fermions are weirder: they correspond to fields that are zero when squared, but are not themselves zero.
So, the three non-gravitational forces that we know about (electromagnetic, weak, and strong) correspond to bosons, of a particular sort: spin 1 gauge bosons. Thus the mathematics of them is, superficially, almost identical. However, there are subtleties that make them behave very differently from each other in practice. These mathematically are characterized by symmetries, and I'll tend to speak loosely and use the words "force" and "symmetry" almost interchangeably below; hopefully this will not be too confusing. Electromagnetism is the simplest: it is what is called an "abelian" gauge theory, which means that the particles carrying electromagnetism (the photons) do not interact with each other. They are also massless. Because of this, light can travel in waves over arbitrarily long distances: looking up at the night sky, you can see some good evidence for this.
The strong force is a lot crazier. It is a "nonabelian" gauge theory; roughly what this turns out to mean is that it has more than one carrier analogous to the photon, and they interact with each other. They are called gluons, and it is possible for two gluons to combine to make a third, or for one to split into two, or for two to turn into two others, or one into three others, or three to combine into one. (This is a rather verbose way to say there is a 3-gluon vertex and a 4-gluon vertex in the theory.) The strength with which they interact with each other is large -- hence "strong" force. This leads to physics that is very different from electromagnetism: gluons can bind together into objects called "glueballs," and they bind quarks (we'll get to those later) together into objects called "mesons" and "baryons," of which the protons you and I are made of are examples.
The weirdest force, though, is possibly the weak force. It's also a nonabelian gauge theory, so once again the force carriers can interact with each other. However, in this case they have mass. The force carriers are called the W plus, W minus, and Z, and they have masses between about 80 and 90 times the mass of a proton. As fundamental particles go, that's really heavy. And it blatantly contradicts the symmetry principle that governs the way these gauge theories work in general. Roughly, what happens is that at high energies (high by the scale of particle physics, small for the real world: roughly 1000 times the proton mass), there is an exact weak symmetry, with 3 *massless* force carriers, again called W's. There is also a force that acts just like electromagnetism, called "hypercharge." Together this is called the "electroweak" symmetry. Then something funny happens at this energy of 1000 times the proton mass, so in the early universe, as it expanded and cooled, there was some sort of phase transition. The hypercharge and weak interactions got all mixed up, and the only remaining symmetry was the electromagnetism that we observe today. The rest of the original symmetry got "broken," but we still see remnants of it in the form of the W plus, W minus, and Z, and the weak interaction they carry.
The punchline is this: this picture of the basic forces has been around since the 70's, and we still have no idea how this transition that breaks the "electroweak" symmetry happened! The best explanation we have is called the Higgs mechanism, and you've probably heard of it if you've read any articles about the Tevatron collider at Fermilab or the Large Hadron Collider that is being built at CERN. It involves yet another boson, but this one isn't a gauge boson like the photon, gluons, W's, or Z. It's what is called a "spin 0" or "scalar" boson. It's a really clever idea, but a somewhat simplistic one, that when tacked onto the Standard Model can explain how this breaking of symmetry happens. Most physicists are convinced that the simplest Higgs idea is not realized in nature, but that something more subtle is going on. Most candidates for something more subtle, though, still involve a particle that looks just like the Higgs boson. This is why the search for a Higgs boson is a big deal. Measuring the properties of one could help guide us toward which more subtle idea is right. Not finding one at all indicates we need a substantial new idea to explain how to get by without it. I'm hoping for the latter: if we don't find what we expect, there's a lot of fun work to be done to try to explain the discrepancy.
There are tons of things I haven't mentioned, like the fermions that make up matter (e.g., quarks that feel the strong force, and electrons that don't), the way the Higgs works, or why we think the Higgs alone isn't enough. But this was probably enough information to digest. I hope this has provided useful background for some of you; I'm not sure how to find the right balance between comprehensibility and amount of information. I would like for more of my audience to understand roughly what I'm talking about in the future when I start posting about some ideas in current research that I'm intrigued by. Feedback is welcome.Posted by Matt at April 20, 2004 01:50 AM